This invention relates to a process for converting hydrocarbon feedstocks to olefins by thermal cracking in high temperature cracking or pyrolysis furnaces. In a particular aspect, the invention relates to such a process including steps for reducing formation of coke in pyrolysis furnaces and associated equipment such as transfer line exchangers.
Olefins such as ethylene and propylene are produced by pyrolysis (thermal cracking) of petroleum gases (butane, ethane, propane, etc.) or of distillates such as naphtha, n-hexane and gas oil. The cracking reactions occur in a thermal reactor comprising tubes or coils inside a high-temperature furnace. During manufacture of olefins, steam can be added to the hydrocarbon feedstocks to reduce hydrocarbon partial pressure to promote production of olefins and to minimize rate of coke deposition. The cracking reactions occur under a reducing atmosphere in which there is substantially no oxygen present.
Pyrolysis tubes are made of metal alloys which can withstand the high temperature and stress-strain relationship of production conditions and that are stable against carburization. A special 25% chrome-20% nickel alloy has been a preferred tube material for reactor coils. Materials having a still higher nickel content, which may be alloyed with minor amounts of carbide stabilizers such as tungsten and niobium (columbium), also are used. Such materials contain catalytic sites which promote the formation of catalytic coke. Typically, other portions of the cracker are made of lower grade steels which are less corrosion-resistant.
Illustrative of olefins production, cracking of petroleum gases or distillates to produce ethylene is accomplished by passing the hydrocarbon feedstock at a positive pressure into a high-temperature cracking furnace composed of a feed section, a convection heating coil section and radiant coil cracking section; cracking the feed in the cracking furnace at a temperature in the range of from about 650.degree. C. to about 930.degree. C. and in the presence of steam, with the steam-to-hydrocarbon ratio typically in the range of from about 0.2:1 to about 1.5:1; pressurizing the cracking furnace effluent in a compressor from substantially atmospheric pressure; and fractionating the pressurized cracking furnace effluent, wherein the pressure profile within the cracking furnace is optimized to maximize the yield of ethylene. Modern ethylene plants are normally designed for near-maximum cracking severity, a term used to describe the depth of cracking or extent of conversion, because of economic considerations. Hence, the production of ethylene from hydrocarbon feedstocks is accompanied by the production of other olefins which entails co-production of many industrially important petrochemicals.
The effluent from the pyrolysis furnace is provided to a transfer-line exchanger (TLE) located adjacent to the furnace. The TLE provides rapid cooling of the heater effluent to prevent further reactions of the effluent from continuing thereby impairing the yield of primary products.
The gaseous effluent from the pyrolysis furnace leaves the TLE at temperatures in the range of 300.degree. C. to 600.degree. C. to enter the separation and recovery portion of the ethylene plant. In this portion, effluent is separated into the desired products by compression in conjunction with condensation and fractionation at gradually lower temperatures. Typically, all the C.sub.2 's are separated from C.sub.3 's and higher. Acetylene must be removed from the C.sub.2 fraction to meet ethylene specifications. Most frequently, the acetylene is eliminated by hydrogenation of the C.sub.2 fraction in the presence of an effective acetylene conversion catalyst.
During operation of olefin furnaces, there is a buildup of coke on the inside of the furnace tubes or coils into which the hydrocarbon feedstocks are fed and on the inside of the tubes or coils in the transfer line heat exchangers into which the effluent from the furnaces is fed. This coke buildup in the ethylene cracking furnace, and in the associated transfer line heat exchanger tubes or coils, reduces heat transfer occurring in the tubes or coils, causes brittlization of the tubes or coils, and reduces the flow rate through the tubes or coils. The coke is removed periodically by steam-air decoking, which, in addition to requiring labor and steam generation facilities and the use of fuel to generate the steam, requires stopping olefins cracking, resulting in olefins production losses due to downtime and other maintenance and utility costs.
To alleviate or minimize the coking rate and to enhance the ethylene production-run length, trace amounts of sulfur compounds such as hydrogen sulfide or ethyl mercaptan (if the feedstock is basically free of sulfur compounds) or antifoulants have been added to the hydrocarbon feedstocks.
Numerous phosphorous-containing compounds have been proposed for continuous or intermittent addition to hydrocarbon or steam feeds to ethylene cracking furnaces. See, for example, Kozman U.S. Pat. No. 3,531,394; Kozman Canadian Patent 935,190; Japanese Patent Application No. 5887/1985; British Patent 985,180; Weinland V. 4,105,540; Kaplan U.S. Pat. No. 4,542,253 etc. Most of these patents suggest pretreatment of the reactor before continuous or intermittent addition to the feed stream. However, these patents do not disclose or suggest using feedstocks free of the phosphorous-containing compound after preconditioning the reactor with phosphorous antifoulants.
These phosphorous compounds are effective coking antifoulants but produce phosphine under ethylene cracking conditions which can deactivate downstream acetylene conversion catalysts.
Phosphorous-containing compounds also cause corrosion problems which some have proposed to alleviate by neutralizing or adding amine stabilizers. To the best of our knowledge, no satisfactory solution to this problem has been found and, in fact, to the best of our knowledge, no phosphorous-containing antifoulants are being used now.
An improved olefin production process is needed which can diminish formation of catalytic coke in furnaces and associated downstream processing equipment. Some of the benefits are longer furnace run length; enhanced flexibility or control of conversion, selectivity and yield; reduced decoking costs; reduced maintenance and tube replacement costs; and significantly improved control of production capability and operating economics.
An improved olefin production process is needed which can diminish formation of catalytic coke while eliminating or limiting the risk of phosphine contamination of downstream hydrogenation catalysts such as acetylene conversion catalyst.
An improved olefin production process is needed which diminishes the amount of coking antifoulant required.
An improved olefin production process is needed which can diminish formation of catalytic coke and can be used with existing olefin plants without significant capital expenditure.
The general object of this invention is to provide an improved olefin production process which can diminish formation of catalytic coke without requiring major modifications to equipment and without significantly departing from olefin plant operating procedures. Other objects appear hereinafter.